In the subsea oil and gas industry, it is often desirable to perform certain fluid processing activities on the sea floor. The processed fluid in subsea hydrocarbon production is typically a multiphase fluid comprising oil and gas and eventually solid matter, that is extracted from an underground reservoir. The processing system is arranged on the sea floor and configured for transport of the process fluid from the reservoir to a surface or land based host facility. Examples of processing systems include fluid pumps (both single phase and multiphase) and compressors (both gas compressors and “wet gas” compressors).
The processing system is subject to variations in pressure in the pumped medium, as well as transitional loads during start and stop sequences. Corresponding measures can be used to prevent process fluid and particulate matter from migrating from the pump interior into a motor housing, and into bearings and seals of the processing system.
In known examples of processing system like pumps or compressors, a motor typically drives a shaft located in a pump section. The motor comprises a housing interior that is hydraulically separated from the pump housing interior by a seal arrangement. A barrier fluid (often a hydraulic fluid) in the motor housing is controlled at a pressure above the internal pressure of the pump. The barrier fluid therefore acts as a barrier that prevents intrusion of process fluid and particles into the motor, the bearing arrangements, and other internal components via the mechanical seals. However, as a result of the pressure difference and intended functionality of mechanical seals, a leak flow of barrier fluid from machine “clean” side to the machine “process” side occurs. This leak flow is referred to as “barrier fluid consumption,” which in a typical setting can be at a rate of about 0-1 liters per hour.
To compensate for an eventual barrier fluid consumption, it is known to have a barrier fluid supplied from a host facility, and leakage compensation as well as pressure control managed from the host facility, usually via an umbilical. As subsea hydrocarbon production sites tend to be installed and operated at increasing depths and step-out distances, the pressure response times and control requirements in the barrier fluid systems increase correspondingly. As a consequence, there is a rising need for a barrier fluid system that operates with improved control requirements and which provides increased reliability in operation.